Android: Calling SuspendingDeferral.Complete() from a Background Thread

[robloo]

Before I file a bug report, I want to see if Uno is behaving as intended and this should be a documented limitation instead.

There is a method similar to what is copied at the end below. This method is called whenever the application is Suspending. The purpose of this method is to save the application state to a working file for use at next startup.

The actual writing of the file is done using another thread in the 'session' which manages and runs all operations like this. Once the file is written to the stream, the given action (last parameter in SaveAsync(), is run as a callback. Within this callback action the deferral keeping the app alive is considered complete.

It is here that calling SuspendingDeferral.Complete() will crash the app

  System.InvalidOperationException
  Message=The application called an interface that was marshalled for a different thread.

Of course the callback action will be run by the background thread that actually wrote the file to the stream. This is easily fixable by adding the below code. However, it isn't necessary on UWP as UWP allows the deferral to be used by background threads.

await CoreApplication.MainView?.Dispatcher?.RunAsync(Windows.UI.Core.CoreDispatcherPriority.High, () =>
{
  deferralType1.Complete();
});

Example code:

        internal static async void SaveWorkingFileAsync(object deferral)
        {
            try
            {
                App.Current.Session.SaveAsync(tempWorkingFileStream, async () =>
                {
                    // From application Suspending event
                    if (deferral is SuspendingDeferral deferralType1)
                    {
                        deferralType1.Complete();
                    }

                    // From application CloseRequested event
                    if (deferral is Deferral deferralType2)
                    {
                        deferralType2.Complete();
                    }
                });
            }
            catch { }

            return;
        }

Stack Trace

 	0xC in Windows.UI.Core.CoreDispatcher.CheckThreadAccess at C:\a\1\s\src\Uno.UWP\UI\Core\CoreDispatcher.cs:66,5	C#
 	0x0 in Uno.Helpers.DeferralManager<Windows.ApplicationModel.SuspendingDeferral>. at C:\a\1\s\src\Uno.UWP\Helpers\DeferralManager.cs:37,5	C#
 	0x1C in Windows.ApplicationModel.SuspendingDeferral.Complete at C:\a\1\s\src\Uno.UWP\ApplicationModel\SuspendingDeferral.cs:28,4	C#
>	0x16B in <App>.StorageManager.<>c__DisplayClass5_1.<SaveWorkingFileAsync<SaveWorkingFileAsync>b__0>d at <App>\Source\StorageManager.cs:250,29	C#
 	0x33 in System.Runtime.CompilerServices.AsyncVoidMethodBuilder.Start<<App>.StorageManager.<>c__DisplayClass5_1.<<SaveWorkingFileAsync>b__0>d> at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:84,17	C#
 	0x27 in <App>.StorageManager.	C#
 	0x5E in <App>.Core.CoreSession. at <App>.Core\CoreSession.Api.cs:581,25	C#
 	0x108 in <App>.Core.CoreSession.OperationExecuter_RunWorkerCompleted at <App>.Core\CoreSession.cs:1267,21	C#
 	0xD in System.ComponentModel.BackgroundWorker.OnRunWorkerCompleted at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/System.ComponentModel.EventBasedAsync/src/System/ComponentModel/BackgroundWorker.cs:81,17	C#
 	0x15 in System.ComponentModel.BackgroundWorker.AsyncOperationCompleted at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/System.ComponentModel.EventBasedAsync/src/System/ComponentModel/BackgroundWorker.cs:36,13	C#
 	0x13 in System.Threading.QueueUserWorkItemCallback.WaitCallback_Context at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/threading/threadpool.cs:1370,13	C#
 	0x73 in System.Threading.ExecutionContext.RunInternal at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:968,17	C#
 	0x4 in System.Threading.ExecutionContext.Run at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:910,13	C#
 	0x2E in System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/threading/threadpool.cs:1341,17	C#
 	0x75 in System.Threading.ThreadPoolWorkQueue.Dispatch at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/threading/threadpool.cs:899,29	C#
 	0x0 in System.Threading._ThreadPoolWaitCallback.PerformWaitCallback at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/threading/threadpool.cs:1261,13	C#

[jeromelaban]

Thanks for the update! @MartinZikmund would you know why this is needed ?

uno/src/Uno.UWP/Helpers/DeferralManager.cs

Line 37 in 56d3ba9

CoreDispatcher.CheckThreadAccess();